1. Technical Field of the Invention
The invention concerns a three-conductor cable for power transmission at a frequency of at least 50 Hz, preferably at least 100 Hz, for example 400 Hz.
In the 400 Hz range, what are referred to as high-frequency power transmission cables are used. They are required for example on aircraft and the like, in order to connect them during stationary periods to a fixed network or a mobile power supply. Because the aircraft's on-board electronics are highly sensitive to current fluctuations, the cable must not generate any damaging asymmetrical voltage drops.
Another domain of application is engine technology. Thus, for example, high-frequency power transmission cables can also be used to power motors for spindle drives (induction/synchronous motors) or brushless DC motors.
2. Description of the Related Art
Known high-frequency cables for frequencies of 400 Hz and above consist of four intertwined component conductors, consisting of three phase conductors and a neutral and/or return line. In this construction, two phase conductors lie adjacent to the neutral and/or return line respectively. Between these two in turn lies the third phase conductor. This asymmetry results in a detrimental inductive voltage drop, which takes on great significance especially in cables which are used at the higher frequency range.
Asymmetrical electrical fields also arise due to the geometry of this known four-conductor cable, which can propagate interference to the immediate environment. The geometry of four stranded single-conductor cables also means that the mechanical position of the arrangement is not clearly defined, and this must normally be resolved by a central element.
However, there are also high-frequency power transmission cables with a symmetrical cable arrangement. These cables do not have the geometry-induced disadvantages of the aforementioned four-conductor cable. In these, the phase conductors run in pairs, stranded about the centrally-disposed neutral and/or return line. This creates a symmetrical arrangement with the neutral and/or return line in the center and six phase conductors stranded symmetrically about this. In this arrangement, two opposing phase conductors are connected with each other. In this construction the return line takes up half the cross-section of the phase conductor. This is a disadvantage when there is an asymmetrical load, as often occurs with wide-body aircraft. Although in operational status the construction has relatively low inductance, it is expensive and usually fairly inflexible. It also requires insulation of two parallel three-phase systems, which means additional expense for high-quality insulation material. Moreover, with this cable the two associated phase conductors must be combined before or inside the plug.
This second type of high-frequency power transmission cable has the disadvantages of being complicated and relatively expensive manufacture. Furthermore these cables have a smaller surface over which the heat losses building up internally can be dissipated into the environment.